Method for producing a high-strength ceramic dental prosthesis

ABSTRACT

Tooth restorations are produced starting from data preferably obtained by optical or mechanical digitizing of prepared teeth and/or teeth of one jaw or of both jaws, intraorally, or from jaw models or partial models of one jaw or of both jaws, extraorally, a subsequent CAD construction of aforementioned restorations being produced in the way that an outer mold, in opposition to the oral cavity is constructed by the primary shaping of the tooth restoration, consisting throughout of high strength ceramic material, taking into account sinter shrinkage.

The invention relates to a method for producing a ceramic toothrestoration such as inlays, onlays, crowns, partial crowns, bridges,implant suprastructures as well as a high strength ceramic toothrestoration made by said method. Ceramic is known in different designsas a material for artificial teeth of all kinds because of its looks andits strength which come nearest to the properties of natural teeth. Whenthe material is appropriately selected, ceramic is at the same timephysiologically harmless. Thereby it is rather the required precisefinal shape which makes up the considerable cost than the materialitself, whereby the desired high strength of the tooth restoration andthe finishing workability of the same in the fired state of the ceramicare opposing each other.

Furthermore, it is generally known to cast the tooth restorations fromspecial metal alloys, whereby, for example, the inner shapecorresponding to a tooth stump will be derived via an impression. Thisprocedure is limited to castable metal alloys.

Furthermore, it is known to copy a pre-manufactured model of the toothrestoration in that a tracer pin is moved along the model, said tracerpin controlling synchronously to its movements a grinding disc and agrinding body, respectively, for working a green body of a toothrestoration (EP 0 267 227 B2). Thereby, in principle, materials ofnearly any desired hardness and strength are workable such as, forexample, ceramic material. However, this requires a very complicatedmechanism and, yet, the working of very delicate details, in particularas concerns indentions and undercuts, is limited due to the finitedimensions of the grinding disc and the grinding body, respectively.

There are also known CAD/CAM methods for three-dimensional milling ofhard fired ceramic solid material for the manufacture of toothrestorations in which information, partly even patient specific ones,such as color, material and occlusion registering are fed into thecomputer. These information are then on-screen processed. Furthermore,an onscreen selection and “modelling” of “crown models”, which arestored in a data base, is carried out (Paper by A. Schmidt, M. Walter,K. Boning “CAD/CAM/CIM-Systeme in der restaurativen Zahnmedizin”,Quintessenz 49, 11, S. 1111–1122 (1998)). Although here, compared to theprior art discussed herein above a greater variety of forms and fidelityin detail are possible without the urgent necessity to manufacturemodels, there remains as a disadvantage the high expenditures in workingthe hard (fired) ceramic material.

Hard tooth restoration green bodies can be brought into the desiredfinal shape by erosion processes such as ultrasonic erosion orelectroerosion, whereby the working tools such as sonotrodes andelectroerosion electrodes have been manufactured before from impressionsand models, respectively, as accurate complementary images of thedesired shape. The electroerosion process is, in principle, limited toelectrically conductive material, whereas the ultrasonic erosion doesnot have this limitation and will be particularly used with ceramicmaterial. By a division along the so-called “equator”, it is possible towork both, the cervical side (the side opposing the jaw) and theocclusal side respectively the incisal side (the side opposing thebuccal cavity) (EP 0 645 195 A1). A combination of both erosionprinciples permits according to a well-devised proceeding a true-to-sizemanufacture of a ceramic tooth restoration or of a metal crown, wherebyeven a set of roughing tools and refined abrading tools can bemanufactured on models manufactured before by galvanic deposition, andprior departures in fitting precision are purported to be overcome (EP 0614 34.4 B 1). The operational sequence will be differentiated whetherceramics or metal is to be worked, whereby the use of the spark erosionrequires forming tools made of graphite in this case. As disadvantagesof these methods remain the long processing time and, above all, as tothe proceedings according to the second mentioned publication, the greatnumber of different succeeding steps of the proceeding which have to besynchronized to one another.

As to the manufacture of the so-called jacket crowns, it is known toisostatically press a pre-manufactured flexible ceramic foil in itsplastic raw state upon a plaster model of the inner form. Said plastermodel has been computer aided modeled before in analogy to the toothstump with an added measure for consideration of the sintering shrinkageof the ceramic material (EP 0 826 642 A1). Since the foil is producedwith a definite thickness which in the course of its transformation onthe model can only slightly be shaped without tearing, this method islimited to the manufacture of crowns and cannot be applied to othertypes of tooth restorations.

Finally, it is known to manufacture the cervical shape of a toothrestoration on a model, for example in plaster, by dry pressing or alsoby slip casting, whereby the tooth restoration is fitted on at least oneprepared surface of a tooth stump or artificial abutments. The occlusalform of the tooth restoration is veneered with dental porcelain fired tothe surface. The model has been manufactured before by means of acomputer controlled milling machine in dependence on a three-dimensionalscanning directly in the mouth or on a casting model, considering thelater sintering shrinkage and a gap for the cement for fixing (EP 0 580565 BI, corresponds to DE 693 20 563 T2, WO 94/27 517 A1). With respectto the occlusal form of the tooth restoration there has only been saidthat the outer surface of the core of the tooth restoration produced inthis way should have a form “near the desired size”, hence, themasticatory surfaces cannot be directly manufacture by said method, butrequire an after-treatment in the fired state. This, however,necessitates further method steps with at least the same demands forfitting precision as well as for appearance and fidelity of detailwhich, in total, at least doubles the expenditures for working. The drypressing of parts made of high-duty ceramics has the disadvantage ofnon-uniform densification in the course of shaping, which leads todefects in the form of a partially porous structure remaining afterfiring and sintering, respectively.

A further known and very complicated and material expensive method formanufacturing ceramic tooth restorations purports to avoid the finalshaping procedure in the ready-sintered (hard) state, which procedurehas been referred to hereinabove and which is particularly criticalowing to the tool wear. It further purports to solve at the same timethe problem of the thin edge portions of a tooth restoration which as aresult of the sintering shrinkage distort or become brittle. To thisend, the negative outer shape and inner shape of a tooth restorationwill be worked out from a cold densifed or partly sintered green bodyform under consideration of the sintering shrinkage. The toothrestoration is shaped out of the chips resulting from the working orfrom a similar material in this mold and ready sintered, whereby saidmold is additionally treated with a mold release agent (WO 96/29951 A2).Thereby one can do without the manufacture of a wax model or a plasticmold, in that, for use in the computer-aided manufacture of the mold,the inlay outer shape as well as the cavity shape and the shape of theprepared tooth stump, respectively, are in-line detected by a sensingwithin the mouth of the patient. The disadvantages of this technicalsolution lie in the long time required for the manufacturing procedureas well as in the expenditures therefor rather than in the quality ofthe tooth restorations manufactured in this way.

Tooth restorations made of aluminum oxide, with additives, if necessary,as well as made of zirconium oxide are known, independent of a certainmanufacturing method for the same (EP 0 593 611 B1).

Starting from the above described prior art, the object of the presentinvention is to provide a method for producing a tooth restoration, inparticular a tight-fitting ceramic tooth restoration, in which underavoidance of a hard-worked ceramic material and with a minimum of methodsteps, the outer shape of the ceramic tooth restoration being inopposition to the oral cavity including the gradually thinning downrestoration edge is manufactured in its final shape, whereby thesintering shrinkage is being taken into consideration.

Starting from data which have preferably been derived from intraoral orextraoral optical and/or mechanical digitizing of tooth stumps and/ordental technological restorations such as inlays, onlays, crowns,partial crowns, bridges, implant suprastructures, respectively from thestructures of all restorations mentioned herein before and/orready-to-use dentures and/or teeth of one jaw or of both jaws and/ormodels or partial models of one jaw or of both jaws, and/or by CADconstructions of inlays, onlays, crowns, partial crowns, bridges,implant suprastructures as well as the structures of all restorationsmentioned herein before, the object is realized according to the presentinvention in that for the primary shaping of the tooth restoration amold, corresponding at least to the outer shape of the toothrestoration, is manufactured which is enlarged and reduced,respectively, by the three-dimensional variation of dimension occurringin the course of the primary shaping. Depending on the form of the toothrestoration to be manufactured, one-part or multipartite molds will beemployed. One-part molds will preferably be used when the toothrestoration to be manufactured is of a geometry free of undercut. Whenthe tooth restorations to be manufactured are of more complex geometriespreferably bipartite or, if necessary, multipartite molds will be used.Undercut shapes will naturally result from the manufacture of partialcrowns, crowns or with tooth restorations for reconstruction of thefunctional masticatory surfaces and/or bridges and/or implantsuperstructures. Thereby the separating planes preferably lie in therange of the anatomic equator of the tooth restoration.

The term “primary shaping” (in German, “Urformung”, as used throughoutthe specification is to be understood as the manufacturing of a solidbody out of unshaped material by providing cohesion thereto, as definedin Dubbels, “Taschenbuch fuer den Maschinenbau”, 19^(th) edition, Berlin1997, page 4, by reference to German standard DIN 8580.

The geometry of the mold defines the outer shape of a ceramic toothrestoration when a one-part mold is concerned, whereas the shaping ofthe inner shape of the tooth restoration can be achieved, abrasingly, byeroding or by cutting, for example, by grinding or milling of the greenbody, the brown body or in the sintered state. The working will bepreferably based on CAD-working data comprising the data base of theinner shape and of the outer shape. Alternatively, the working is alsopossible by conventional technologies such as copy grinding and copymilling, respectively. The three-dimensional association of the shapesin the working machines is preferably carried out via reference planesand clamping reference surfaces. Alternatively, a positioning by theworkpiece is possible.

In the case of a multipartite form, the geometry of the mold defines theouter shape and in most cases also the inner shape of the toothrestoration. Thereby the outer shape of the tooth restoration will beworked as a negative, whereas the inner shape of the tooth restorationwill be worked as a positive. In the case of crowns or partial crowns orinlays, the cervical parts of the restoration are defined preferablytogether with the inner shape. In the case of FPDs, there also will bedefined, preferably together with the inner shape of the bridge, thebasal surface of the pontic. The three-dimensional association of thepartial shapes to each other is preferably carried out via referenceplanes and clamping reference surfaces.

By the construction of the shape of the tooth restoration, whichpreferably is accomplished by CAD/CAM technologies, it is possible tocomputer aided simulate and to compute the three-dimensional dimensionvariations occurring in the course of primary shaping (Urformung) foreach restoration. The digitized data of the prepared teeth serve as abasis for the dental CAD/CAM technologies obtained by intra-oral orextra-oral, optical or mechanical measuring of the teeth, of models orpartial models of one jaw or of both jaws with or without the detectionof the: spatial association of the upper jaw and the lower jaw.Optionally the digitizing data of the neighboring teeth are utilized forthe design of the approximal portions of the inlays, onlays, crowns,partial crowns, bridges, and implant suprastructures and of thestructures, respectively, of all restorations mentioned above as well asthe digitized data of the antagonistic teeth in the reconstruction ofthe functional masticatory surfaces of the restorations to bemanufactured. It is possible to optimize the quality of thereconstruction of the masticatory surfaces of the inlays, onlays,crowns, partial crowns, bridges, and implant suprastructures and of thestructures, respectively, of all restorations mentioned above byutilizing the digital data of the upper jaw and the lower jaw incombination with the data of the movement pattern of the mandibularjoints.

Into the simulation of the three-dimensional dimension variations of thematerial in the course of the primary shaping, there are integrated thedata of the prepared teeth, the geometry of the constructed inlays,onlays, crowns, partial crowns, bridges, and implant suprastructures andof the structures, respectively, of all restorations mentioned above,furthermore the layer thickness of the dental fixing material as well asthe technological parameter of the mold material, of the ceramicmaterial, and of further procedural performance influencing factors ofthe respective primary shaping process. Thereby one-part molds arepreferably made from block material. In the case of more complexgeometries or under-cut outer shapes of the tooth restorations to bemanufactured, a separation of the mold into at least two parts isnecessary. To this end, special pre-manufactured blanks for partialmolds which preferably consider the separation planes and the inletgates or injection ports for the respective shaping method are used. Theindividual partial molds are bi-uniquely associated to each other viareference planes and clamping reference surfaces. Thereby the machiningof the partial molds is performed by three-axial or multi-axialmachinery.

The material-used for the manufacture of tooth restorations preferablyare ceramics, in particular high strength and high-purity ceramics(Al₂O₃, partly stabilized or fully-stabilized ZrO₂), dispersed ceramics)of mean grain size within the grain structure <1 μm, but also metals.For improving the esthetic appearance of the tooth restoration, both,high strength transparent ceramic material and such ones being coloredin accordance with the 16 so-called “vita-colors” are utilized. It isalso possible to use gradient material for optimizing the optical and/ormechanical properties. A possible esthetic optimizing of therestorations to be manufactured preferabiy is achieved by means ofdopants in the ppm-range.

The primary shaping of the inlays, onlays, crowns, bridges, and implantsuprastructures and of the structures, respectively, of all restorationsmentioned above is preferably carried out by ceramic slip casting orceramic injection molding.

By virtue of the present invention it is possible to have a principallymore favorably priced shaping of the tooth restoration, in that theshaping by cutting is shifted from the hard-machining of the ceramic tothe considerably softer material of the mold for the primary shaping,such as plaster. Thereby one has to consider that the tooth restorationaiming at the reconstruction of the occlusal (masticatory) surfaces ischaracterized in particular by the composite and complex shape of theouter shape, whereas the already commonly used computer aidedmanufacture of the inner shape is comparatively simple.

The tooth restoration is realized by maintaining the strength inherentin the respective material (“as fired”).

By virtue of the use of the present invention, the manufacture of themetal-free tooth restoration such as, for example, inlays, onlays,partial crowns, crowns, bridges, and implant suprastructures as well asbridges with functional, masticatory surfaces becomes possible whichsatisfy without any reworking the esthetic and biological requirementsto a definitive prosthetic treatment.

Further details and features of the method according to the presentinvention will become apparent from the following description of theembodiments in connection with the drawings. There is shown in:

FIG. 1 a representation of the principle proceeding in manufacturing atooth restoration according to the inventional method,

FIG. 2 a representation of a one-part casting mold according to theinventional method with an outer shape of a crown structure,

FIG. 3 a representation of a casting mold according to the inventionalmethod with an inner shape of a crown structure,

FIG. 4 a perspective representation of a bi-partite casting moldaccording to the inventional method,

FIG. 5 a perspective representation of a casting mold according to theinventional method with an outer shape of a crown including a functionalmasticatory surface,

FIG. 6 a representation of a casting mold according to the inventionalmethod with an inner shape of a crown,

FIG. 7 a representation of a bi-partite casting mold of a crown with afunctional masticatory surface according to the inventional method,

FIG. 8 a representation of a bi-partite casting mold of a bridgestructure.

The sequence of the inventional method steps (FIG. 1) permits anoptimizing with respect to the costs by a splitting up of the methodstep into such being centralized and such which have to be carried outdecentralized. CAD/CAM service provider and specialized dentaltechnicians, respectively, who operate for a plurality of clients,cooperate, for example, in the following suitable, but of course notcompulsory, splitting up of the operations:

-   -   Inventional decentralized dental treatment with preparation;    -   Shaping and manufacture of precision models and the optical        digitizing of the same and a direct intra-oral digitizing,        respectively;    -   Central computation of the geometric shape of the mold for the        slipcasting under consideration of the simulated shrinkage        behavior;    -   Central generation of the CNC-program for working the individual        molds for the slipcasting as well as the manufacture of the        same;    -   Central casting and sintering of the tooth restoration;    -   If necessary, decentralized esthetic individualizing of the        tooth restoration.

The forms in FIG. 2 to 7, illustrate the system of reference planes andclamping reference surfaces by the respective X-, Y-, and Z-axes. Theseforms serve to realize the method for manufacturing ceramic toothrestorations as well as high strength ceramic tooth restorationsmanufactured by said method. These forms are composed of the one-partmold or the multipartite mold (FIG. 3) which is comprised of the outermold half 1 and the outer mold half 5, 10, 18, respectively, and theinner mold half 3, 8, 13, 19 for defining the outer shape 2 and theouter shape 7, 11, 16, and 20, respectively, and the inner shape 9, 14,17, 21 of the restoration to be manufactured, furthermore, of the systemof casting channels 6 as well as the separation planes 12 and 15,respectively. Fig. 2 shows an outer shape free from undercut which, inprinciple, permits a one-part mold for defining the outer shape 1 of therestoration to be manufactured. Together with the inner mold half 3shown in FIG. 3 defining the inner shape 4 of the restoration, these canbe assembled to a multipartite mold (FIG. 4). Furthermore, FIG. 4 showsthe casting channel 6 as well as the separation plane. More complexseparation surfaces 12 and 15, respectively, result from an increasingcomplexity of the geometry of the restorations to be manufactured (FIG.5, 6, 7). FIG. 8 shows the outer shape and the (20 and 21, respectively)of a bridge.

The invention will be further explained as concerns the material byvirtue of two embodiments.

EXAMPLE OF EMBODIMENT 1

A drum mill is charged with 500 g aluminum oxide of a grain size ofd₅₀=0.2 μm together with 0.05 weight % magnesium carbonate, and 1.5weight % of a known liquefier for preparing a casting slip as well aswater. The weight ratio of material to be milled:milling bodies is 1:6.After a milling time of 24 hours the slip is removed. Its solids contentis 71.0 weight %. For manufacturing a crown, this slip is filled intothe casting mold which has been modeled according to the scheme of theillustration. After a solidification time of about 30 minutes the crownis demolded and, after appropriate drying it will be fired at a hold-uptime for 2 hours at 1350° C.

EXAMPLE OF EMBODIMENT 2

A batch of 500 g of 80 weight % aluminum oxide and 20 weight % partlystabilized zirconium oxide of a grain size of d₅₀=0.3 μm are processedin analogy to embodiment 1. The only difference is that sintering iscarried out at 1400° C.

LIST OF REFERENCE NUMERALS

-   1 outer mold half-   2 outer shape-   3 inner mold half-   4 inner shape-   5 outer mold half-   6 casting channel-   7 outer shape-   8 inner mold half-   9 inner shape-   10 outer mold half-   11 outer shape-   12 separation plane-   13 inner mold half-   14 inner shape-   15 separation plane-   16 outer shape-   17 inner shape-   18 outer mold half-   19 inner mold half-   20 outer shape-   21 inner shape

The invention claimed is:
 1. Method for producing high strength ceramicdental restorations including inlays, onlays, partial crowns, crowns,FPDs and superstructures for implant retained prosthesis, that can befitted onto at least one prepared surface of a tooth stump or artificialabutment, said restoration having an internal geometry determined by aborder surface occurring between said prepared surface and saidrestoration, and an external geometry, said method comprising makingdata available to define said internal geometry by means of digitizationof the prepared surface and using said data to form the inner shape ofsaid restoration and using digitization to obtain data defining anegative form of neighboring and antagonistic teeth and masticatoryfaces of said restoration as said external geometry, and using saidlatter data to construct a mold, and utilizing said mold to form theouter shape of said restoration, said method steps resulting in theforming of said dental restoration consisting throughout of said highstrength ceramic for its final shape.
 2. Method as claimed in claim 1,further comprising producing a reproduction of physiological naturalmasticatory faces having respective depth of fissures and fissure radiiby slip casting in a primary shaping procedure.
 3. Method as claimed inclaim 1 or 2, wherein the outer shape of the restoration is reduced byan amount corresponding to a subsequent ceramic veneering.
 4. Method asclaimed in claim 1 or 2, wherein the high strength ceramic material isdifferently colored by means of dopants to simulate coloring of anatural tooth.
 5. Method as claimed in claim 1 or 2, wherein the highstrength ceramic material is transparent.
 6. Method as claimed in claim1 or 2, wherein in the course of primary shaping the ceramic dentalrestoration is given, taking into consideration the layer thickness ofdental bonding material, a final inner shape in opposition to the jawand to the prepared surface, respectively, said final inner shape beingobtained by an impression.
 7. Method as claimed in claim 1 or 2, whereinshaping of the inner shape is carried out by machining the ceramic as agreen body, a brown body or in the sintered state.
 8. Method as claimedin claim 1, wherein the primary shaping is carried out by ceramicinjection molding.
 9. Method as claimed in claim 1 including the step ofusing said data to form the inner shape of said restoration in theconstruction of said mold and subsequently utilizing the mold for theprimary shaping of the restoration including both its inner and outershapes.